There are no things that help anxiety other than cutting down on caffeine and some other additives in food.
If they are truly having issues then they need counseling to find the root of the anxiety. Once they find that and work through it they won't have anxiety any more. Period. Anxiety is caused by something external.
If they have other things going on that are biological they need medications to control the levels of chemicals in their body.
Like diabetes. One with blood sugar issues can manage that by eating less foods with abundant sugar, even without ever eating or drinking any products called sugar free anything.
BUT once that person gets to a point or if they have a different type of diabetes diagnosis they have to take medication to control their body better. Even if they eat completely healthy and take dozens of supplements they are still diabetic and require medication to fix it.
My point is if the anxiety is a mental health issue then counseling is the only thing that will cure it, that will probably take a year or so to get to the root and work through it and be fixed, no more anxiety.
If this is biological and they have a chemical imbalance then medication prescribed by a physician will be the only course they can follow.
Trying to be a pharmacist and doctor won't help your kids. They need to either find out why and fix it or get a medication that prohibits the flight or fight system from kicking in for no reason.